Adaptive Cruise (i.e., speed) Control (ACC) systems operate much like conventional Cruise Control systems, with the added capability of being able to sense in-path vehicles and to slow the ACC equipped vehicle in response. An ACC equipped vehicle thereby allows its operator to automatically control the vehicle speed, as with conventional Cruise Control, without the necessity of having to deactivate and reactivate control whenever slower traffic is encountered.
As is well known in the art, existing ACC methods and systems use a forward looking range sensor such as radar to sense an in-path vehicle (which may also be referred to as a sensed target or primary target). Based on the radar sensor information, such ACC methods and systems then determine the range and relative velocity (or range rate) of the sensed in-path vehicle. Using the range and range rate, the speed of the ACC equipped vehicle is controlled to maintain a selected following interval between the ACC equipped vehicle and the sensed in-path vehicle. The speed of the ACC equipped vehicle is typically controlled by automatic control of the vehicle throttle actuator. In more advanced ACC methods and systems, vehicle speed may also be controlled by automatic control of vehicle brake actuators. Such ACC methods and systems have the ability to apply a moderate degree of braking to the vehicle to achieve further vehicle deceleration (i.e., in addition to vehicle deceleration achieved via throttle control) in response to an in-path vehicle.
Existing ACC methods and systems, however, generally do not take into account driving surface conditions. More particularly, the time necessary for a vehicle operator is to slow a vehicle to a selected speed is optimal on those driving surfaces having high coefficients of friction, such as those provided by dry, concrete or asphalt pavement. That time generally increases when the driving surface coefficient of friction deceases, such as when the driving surface is wet, or is covered by snow or ice. Existing ACC methods and systems, however, control the vehicle speed to maintain the following interval selected (either by the vehicle operator or by default) regardless of the driving surface coefficient of friction.
Thus, there exists a need, in an ACC system, for a method and system for automatically adjusting the following interval of the ACC equipped vehicle based on the driving surface coefficient of friction. Such a method and system would determine a driving surface coefficient of friction based on a driven wheel speed of the ACC equipped vehicle, and adjust the selected following interval for the ACC equipped vehicle based on the driving surface coefficient of friction.